1. Field of the Invention
The present invention relates to a spring-return type actuator for valves, particularly for a ball valve, butterfly valve, etc. which is opened or closed by its partial rotation, and specifically relates to a spring-return type actuator comprising a piston cylinder mechanism and a spring mechanism which are disposed in series and fulfilling its function by the reciprocation of a piston which is moved in one direction by the pressure of a fluid such as air, oil, etc. introduced into the piston cylinder mechanism and in the other direction by the resilient force of a spring accommodated within the spring mechanism.
2. Description of the Prior Art
Actuators of the aforementioned type have heretofore been proposed such as, for example, in U.S. Pat. Nos. 4,337,691 and No. 4,355,566. FIG. 1 is a lateral cross section illustrating a prior art spring-return type actuator, FIG. 2 a partially sectioned front view thereof, and FIG. 3 a partially cutaway front view thereof. A spring means 1 has a spring 2 accommodated therewithin. The spring 2 which has been extended to the fullest extent within the spring means 1 as illustrated in FIG. 1 is still kept compressed as accumulating large resilient force. For this reason, when nuts 6 which have fastened rods 5 connecting the opposite side walls 3 and 4 of the spring means 1 as shown in FIG. 2 are loosened and removed for the purpose of disassembling the spring means 1, the side wall 3 is suddenly sprung open by the large resilient force of the spring 2 and the spring 2 simultaneously bursts out of the spring means 1, thereby giving rise to a possibility of injuring the workers and damaging the surrounding articles.
In order to avoid this, at least two long threaded rods 7 have been used in disassembling/assembling the spring means 1. With reference to FIG. 3, though showing a single long threaded rod 7, the spring means 1 is disassembled by loosening and removing a pair of diagonal nuts 6, replacing the corresponding connecting rods 5 with the long threaded rods 7, and loosening and removing the remaining nuts 6. In assembling the spring means 1, at least two long threaded rods 7 are set in position, then the nuts 6 are driven about the long threaded rods 7 to gradually compress the spring 2, and subsequently the long threaded rods 7 are replaced with the connecting rods 5. Since the threaded rod 7 is long, it is difficult to handle. Further, if the thread portion of the long threaded rod 7 should be damaged, the disassembling/assembling work will be difficult to conduct. Furthermore, since the actuator in use undergoes vibration every one stroke of the piston, the repeated vibration may possibly cause the nuts 6 to be loosened and finally removed, with the result that the side wall 3 is spring out and the spring 2 is burst out of the spring means 1 resulting in injury to the workers and damage to the surrounding articles.
What is worse, it is possible that workers may be unaware of or forget the necessity of using the long threaded rods 7 and thus they may loosen the nuts because the spring means 1 has a construction such that it can be disassembled without using the long threaded rods 7. It will be very difficult to keep all workers aware of the necessity of using the long threaded rods 7, make them pay attention to this necessity at all times, and always place the long threaded rods 7 in the vicinity of the actuators.
In view of the difficulty mentioned above, it has heretofore been tried that at least two long threaded rods 7 are mounted in advance on the side wall 3 as projecting from the side wall 3. In this case, however, the projecting long threaded rods 7 require the actuator to occupy a large space, hinder the workers from handling the actuator, and sometimes injure the workers at their thread portions.
Thus, the conventionally used long threaded rods are insufficient as a means for preventing the accidental danger from occurring, and the prior art spring-return type actuators entail the drawbacks as described above.